Various rotation speeds used for a speed ratio control of a continuously variable transmission such as a vehicle speed, and input/output rotation speeds of the transmission are detected by counting pulse signals which a hall element emits corresponding to the passage of a gear formed on a rotation axis.
However, when the vehicle speed is very low such as when the vehicle is starting, the speeds detected by such a method are not sufficiently accurate for use in the control of speed ratio.
Tokkai Hei 9-53716 published by the Japanese Patent Office published in this regard in 1997 discloses a speed ratio control of a toroidal continuously variable transmission wherein feedback control and feed forward control (open loop control) are combined.
In other words feed forward control is applied to speed ratio control when the detection accuracy of various rotation speeds is low, and feedback control is applied after the detection accuracy reaches a predetermined accuracy level.
The above prior art also discloses to learn a control value in feedback control and modify a target speed change ratio of feed forward control using a learned control value in order to prevent a speed change shock from occurring when the control method is changed over.